Method of sintering maraging steel with boron additions



3,451,809 METHOD OF SINTERING MARAGING STEEL WITH BORON ADDITIONS Rasipuranl Subramanya Kalyana Raman, Madras, India, and Victor Allen Tracey, Solihull, England, asslgnors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware N Drawing. Filed Mar. 8, 1968, Ser. No. 711,482

Int. Cl. B22f 3/16; C22c 39/20, 39/54 U.S. Cl. 75-205 3 Claims ABSTRACT OF THE DISCLOSURE A mixture of finely divided powders for compaction and sintering to form components of maraging steel composition contains from 0.02% to 0.20%, by weight, of boron. The boron functions to reduce linear shrinkage during the sintering operation.

The invention is directed to a novel composition for the constituent powders for producing maragining steel components by powder metallurgy techniques; compacts of the constituent powders being characterized by great dimensional stability during the sintering operation.

The maragining steels have been the subject of rather intense interest and extensive development during recent years. The term maraging is derived from the circumstance that the final strength of these steels is determined by the development of martensite upon cooling from the austenite phase, and subsequent age hardening of the martensite.

A wide range of compositions are known which are capable of undergoing maragining processes. Thus, various compositions are disclosed in Bieber US. Patent No. 3,093,518, Decker et al. US. Patent No. 3,093,519, Sadowski et al. US. Patent No. 3,132,937, Decker et al. US. Patent No. 3,132,938, Floreen et al. US. Patent No. 3,166,406, etc. These patents disclose compositions wherein the matrix includes the elements nickel and iron and at least one of the elements cobalt, molybdenum, chromium, titanium and aluminum, as well as other elements.

It is clearly desirable to produce articles and components of maragining steels by the well known powder metallurgy processes in which a powder mixture of the desired composition is compacted and thereafter sintered. Machining and other finishing costs can be minimized by such processing and, thus, economical use of materials is effected.

One of the major problems relating to the manufacture of maragining steel products is the control of the dimension of the compacts during sintering, for it is found that during sintering, compacts of maragining steels sustain substantial shrinkage. Linear shrinkage on sintering may amount to as much as 3% or more in the maraging steels.

It has now been discovered that the inclusion of a small amount of a special agent in a maraging steel powder mixture reduces shrinkage of maragining steel powder metal compacts.

It is an object of this invention to provide means for reducing shrinkage during sintering of maraging steel powder metal compacts.

Other objects and advantages of the invention will become apparent from the following description.

In accordance with the invention, a body of maraging steel is formed by sintering a compact, the constituent powders of which include, by weight, from 0.02% to 0.20 boron.

A preferred range of boron additions to the maraging 3,451,809 Patented June 24, 1969 steel constituent powders, within the broader range defined above, is, by weight, from 0.05% to 0.15%. It should be understood that the boron addition of this invention may be made to the constituent powders of a broad range of maraging steels. Such maraging steels may contain, by weight, from 8% to 27% nickel, e.g. from 10% to 20% nickel, up to 18% chromium, e.g. from 2% to 8% or from 8% to 14%, and at least one ageing element selected from the group consisting of up to 30% cobalt, e.g. 1% to 20% cobalt, up to 16% molybdenum, e.g. from 2% to 15% molybdenum, up to 6% copper, e.g. from 1% to 4% copper, up to 3% titanium, e.g. from 0.05 to 2% titanium, up to 3% aluminum, e.g. from 0.05% to 1.5% aluminum, up to 5% manganese, e.g. from 0.05% to 2% manganese, up to 3% silicon, e.g. from 0.2% to 2.5% silicon, and up to 0.1% carbon, e.g. up to 0.03% carbon, and the balance essentially iron.

Elements which may be present as auxiliary hardeners or for other purposes in the alloys of this invention include, in a total amount of up to 7%, by weight, up to 1% beryllium, e.g. from 0.2% to 0.5% beryllium, up to 6% tungsten, e.g. from 1% to 3% tungsten, up to 3% in total of one or both of niobium and vanadium, and up to 1% of zirconium. Although zirconium appears to cooperate with boron in reducing shrinkage on sintering, it has an adverse effect on the impact strength of the steel and therefore preferably it is not present in amounts exceeding 0.5 and most advantageously is absent.

Small amounts of impurities such as, for example, sulfur and phosphorus, will also be present, but the total amount of such impurities should not exceed 0.05 by weight.

One group of maraging steels to which the invention applies includes, by weight, from 10% to 20% nickel, up to 8% chromium and at least two elements in a total amount of at least 2% selected from the group consisting of up to 10% cobalt, up to 10% molybdenum, up to 3% titanium, up to 6% copper, up to 3% silicon and the balance essentially iron with small amounts of incidental impurities. A boron addition of from 0.2% to 0.2%, or preferably from 0.05% to 0.15%, is made to the constituent powders of the alloys just defined.

In a preferred composition within the broad alloy range above, a boron addition of from, by weight, 0.02% to 0.2% is made to a maraging steel comprising from 16% to 20% nickel, from 6% to 9% cobalt, from 3% to 6% molybdenum, from 0.1% to 0.8% titanium and the balance essentially iron except for small amounts of incidental impurities.

In carrying the invention into practice, the boron is mixed with the powders to be compacted and sintered in any convenient form. It may be added as finely-divided elemental boron, but in order to obtain the finest possible dispersion it is preferably added as a boron compound, for example, as finely-divided ferroboron, nickel-boron, Lnanganese boride, nickel-chromium-boron, or zirconium oride.

The iron constituent of the powder employed in making the compacts of the invention may be in the form of a powder substantially all of which will pass through a mesh British Standard Screen (B.S.S.) sieve and, more preferably, a 300 mesh B.S.S. sieve. The shrinkage on sintering depends to some extent on the size of the iron powder forming the major constituent of the compacts, and increases as the particle size decreases. The nickel constituent is preferably added in the form of fine nickel carbonyl powder (average particle size of less than 9 microns). The cobalt and molybdenum constituents are zhrough a 300 mesh B.S.S. sieve. The titanium and silicon :onstituents are added in the form of finely ball-milled naster alloys, such as alloys containing 76.5% nickel- 23.5% titanium and 20% nickel-80% silcon, respectively, which are ground to pass through a 300 mesh B.S. sieve.

mined, the means of several measurements being taken. The densities of the sintered compacts were also determined.

The results set forth in Table II below show the effect of the boron additions on the behavior of compacts of 5 The boron add1t1on may be 1ntroduced, for example, as maragmg steels on sintermg. The steels identified by z1rcon1um d1bor1de powder havmg an average particle an asterisk in the first column were made using cars1ze 1n the range of 5 to m1crons. Other constltuents bonyl iron powder, and all the others with the coarser added w1ll also be 1ntroduced 1n the form of fine powders. sponge iron powder.

TABLE II B addition Linear Steel Added Nominal shrinkage Stress (0.2% Elong. on 1 No. as (percent) (percent) Density (g./cm. along), t.s.i. (percent) NiB 0 3.0 7.65 N1B 0. 005 2. s1 7. 64 40. 8 3. 0 N1B 0. 01 2. 50 7. 61 52. 0 2. 5 N B 0. 05 1. 38 7. 20 51. 4 1. 6 N113 0.10 0.85 7. 52. 5 1. 0 N1B 0. 15 1. 03 7. 17 55. 5 0. 6 MB 0. 20 1. 34 7. 31 56. 9 0. 7 z B 0.10 0.57 7.07 63.7 0.6 1 T1134 0.10 1.2 7.3 54 2.3

NiB 0 1.60 7. 36 NiB 0. 005 1. 41 7. 45. 1 N 13 0. 01 1. 26 7. 32 46. 0 N1 B 0. 05 1. 17 7. 27 48. 4 N13 0. 10 0. 24 7. 07 47. 5 NiB 0.15 0. 60 7.11 51.1 N 1; 0. 20 0. 92 7. 19 53. 6 N1B 0 2.4 7.5 49 N1B 0.10 1.4 7.3 55

3 NiB 0 2. 4s NiB 0.10 1.70

- NiB 0 3. NiB 0.10 2.74

5 Nil? 0 0.9 5 NiB 0.10 0.5 5* NiB 0 2.7 5* NiB 0.10 0.0

Examples These results for Steels 1 and 2 show that the shrink- For the purpose of illustrating the advantages of the 40 age progressively decgeases as more boron 1s aldded, reachinvention, sintered body of a number of different maragmg g at a out 7 l t en mcreacsles ing steels were made by sintering compacts of the conagam 1 s p 1S g er Increased The 3- stituent powders admixed with varying amounts of boron. grease i 3 f accompanle t a$ W0u1d be flf h The nominal compositions in weight percent of the steels g 'i f i m the Smtered bodles' if e to which the boron was added are set forth in Table I, i t 8 0 enhance? the e the balance in each case being iron and impurities. D O but lmpnrs h Strengh and for the h1ghest toughness z1rc0n1um 1s preferably not added. TABLE I The sintered maraged steels are somewhat stronger but Percent less ductile with increasing boron content.

The results obtained for maraging Steels 3 4 and 5 St lN Ni 00 M0 Ti Al Cu Si (Jr ee 0 confirm the fact that an add1t1on of 0.10% boron sub- The appropriate constituents in the form of finely-divided metallic nickel, iron; molybdenum, cobalt and copper, the iron being in the form either of sponge iron powder having a particle size less than 300 mesh B.S.S. or of carbonyl iron powder having a particle size of about 7 microns; finely-divided alloys of titanium and silicon with nickel; and the finely-divided boron carrier; were thorougly mixed with a small proportion (0.75% by weight) of lithium stearate as lubricant by tumbling for two hours in a cone-type mixer and compacted under a pressure of 50 long tons per square inch to compacts measuring 5 x x inches. After heating for hour at 500 C. in dry hydrogen (dew point-40 F.) to remove the waxy lubricant, the compacts were heated to 1300 C. for 1 hour in vacuum to effect sintering. The sintered bodies of the maraging steels Nos. 1 to 5 were then heated at 480 C. for 3 hours to bring about aging. No further volume change took place during this aging treatment. The dimensions of the compacts were measured before and after sintering and the linear shrinkage thus deterstantially reduces shrinkage during sintering. Comparison of the results obtained for Steels 2 and 5 with the two ditierent kinds of iron powder shows that while use of the very fine carbonyl iron powder increases the magnitude of the shrinkage, the beneficial effect of a boron addition is still obtained.

In order to show the unique effect of boron in reducing sintering shrinkage in maraging steel compacts, boron additions were made to the constituent powders of the three non-maraging steels set forth below in Table III:

TABLE III Percent In contrast to the favorable effect of boron on the shrinkage of the maraging steels indicated in Table II, the results set forth in Table TV show that similar additions of boron to the non-maraging nickel steels A, B and C have little eifect on the shrinkage, in the cases of Steels B and C the shrinkage even being slightly increased.

TABLE IV B addition Linear Added Nominal shrinkage Density Steel as- (percent) (percent) (g./cn1.

The decrease in sintered density of the boron-containing compacts in Table II shows clearly that this invention is not to be confused with so-called liquid phase sintering, in which compacts contain additions in order to form substantial amounts of low-melting phases that melt during sintering and greatly increase the bond area between the particles and reduce the porosity of the compacts so that the density of the sintered bodies is increased. Although it has been proposed to add boron to steel c0mpacts for this purpose, much larger additions are required than are made in the present process.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview of scope of the specification and appended claims,

We claim:

1. In a method for making maraging steel powder metal components including the steps of compaction of the constituent powder mixture to a body of predetermined configuration, and sintering the compacted body at an elevated temperature; the improvement comprising introducing into the constituent powders, prior to the compaction step, from 0.02% to 0.20%, by weight, of boron whereby the shrinkage of the compacted body during sintering is substantially reduced.

2. A method in accordance with claim 1 wherein the boron addition amounts to from 0.05% to 0.15%, by weight.

3. The method of claim 1 wherein the constituent powders comprise, by weight, from 8% to 27% of nickel, up to 18% chromium, and at least one aging element selected from the group consisting of up to 30% cobalt, up to 16% molybdenum, up to 6% copper, up to 3% titanium, up to 3% aluminum, up to 5% manganese, up to 3% silicon, up to 0.1% carbon and, in a total amount of up to 7%, by weight, up to 1% beryllium, up to 6% tungsten, up to 3% in total of niobium and vanadium, and up to 1% of zirconium, and the balance iron except for small amounts of incidental impurities.

References Cited UNITED STATES PATENTS 2,831,243 4/1958 Thomson 29-1825 3,177,564 4/1965 Reynolds 29-1825 BENJAMIN R. PADGETT, Primary Examiner. ARTHUR J. STEINER, Assistant Examiner.

U.S. c1. X.R, 29-1825; -314 *ggggy UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,451,809 Dated June 2 1, 1 g69 Inventofls) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, lines 23, 27, 35, 46, 53, 55 8c 61, for "maragining",

read maraging--.

Column 1, line 7l, for "0.20", read "0.20%".

Column 2, line 21, for "0.2%", read "0.02%".

Column 2, line 40, for "0.2%" first occurrence, read --0.02%--.

Column 3, line 4, for "silcon", read --si1icon--.

Column 3, line 5, for "3.5.", read --B.s.S.--.

Column 3, line 41, for "body", read "bodies".

Column 3, 1 line 67, for "5 x 5/8 x 5/8 inches. After heating for hour at 500C. read --5 x 5/8 x 5/8 inches After heating for hour at 500C.-.

Column l, line 1, for "means", read --mean--.

Column line 5, for "of boron additions of the behavior of compacts of",read "of the boron additions of the behavior of compacts of the--.

SIGNED ANI QFALED (SEAL) vim-ml r. m. hng Offioor Oomiasiom of Patents 

